This invention pertains to hydromechanical transmissions utilizing both mechanical and hydrostatic drives which are connectable to elements of a plurality of planetary gear sets for plural modes of operation and with clutch and brake means operable with the structure in different relations to provide the plural modes of operation.
Hydromechanical transmissions have the potential to provide increased economy and productivity when used with vehicles, such as line haul trucks of the trucking industry. However, in the past, hydromechanical transmissions have had some undesirable characteristics as compared to a standard gear transmission which normally cruises in a locked, straight-through drive and exhibits high efficiency and long life because few parts are subject to wear and fatigue. The hydromechanical transmissions have had large amounts of power transmitted through several gear meshes and through the hydrostatic units which has resulted in loss of efficiency and loss of long operating life and reliability. Additionally, there have been parasitic power losses, due to auxiliary pumps and clutch and gear spinning losses, as well as an inability to operate at an "overdrive" speed ratio at normal cruise conditions, so that engine speed can be reduced to its maximum economy range, while still retaining normal drive line components. Further, present hydromechanical transmissions can exhibit less than ideal mode shift characteristics, due to problems associated with hydrostatic unit volumetric efficiency.
In applying the hydromechanical transmission to vehicles other than line haul trucks, such as transit mixers, buses, dump trucks or refuse haulers, there are a wide variety of operational requirements and, in such an environment, a hydromechanical transmission has been subject to some disadvantages when compared with conventional shifting transmissions. Normally, there have been fixed values for input speed, output speed, and maximum output torque, while, in manual gear transmission, a wide range of input speed can be accepted, with change of relative speed and torque ratios in both low and high gear ratios. Additionally, use of hydromechanical transmissions in the last-mentioned type of vehicle, results in the hydrostatic transmission working at high pressure when operating at low ground speed, with resulting lower efficiency of the hydraulic units, which results in power loss, increased cooling requirements and more noise generation. Also, an unusually high number of vehicle starts at high pressure can reduce the life of the hydrostatic units.
A number of prior art patents relate to hydromechanical transmissions. The patents show: a two-mode transmission utilizing two planetary gear sets and a clutch and a brake; hydromechanical transmissions which have had their components in-line; a three-mode transmission using one planetary gear set with plural clutch and brake means; and a three-mode transmission having at least two planetary gear sets with three modes and plural clutch and brake means. This patent art does not disclose the features and objects of this invention, as set forth hereinafter.
An application of George A. Schauer, owned by the assignee of this application, entitled A PLURAL MODE HYDROMECHANICAL TRANSMISSION WITH SYNCHRONOUS SHIFT, Ser. No. 21,401, filed Aug. 1, 1977, now abandoned, discloses a four-element planetary gear system used in a three-mode hydromechanical transmission with synchronous shift between modes and selective drive of a sun gear.